1. Field of the Invention
The present invention relates to an injection molding method of metering a molding material of a synthetic resin and then injecting it into a cavity in a mold by the forward movement of a screw in an injection heating cylinder to mold it into a required molded article.
2. Background Art
Conventional injection molding method using common synthetic resin (hereinafter simply called resin) comprises a metering process in which by use of a machine including a screw in its injection heating cylinder, the resin supplied into the injection heating cylinder is made plasticated under a back pressure while rotating the screw and stored in the head portion of the injection heating cylinder by the backward movement of the screw associated with the plastication, and an injecting process in which the metered resin is injection charged into the cavity of the mold by the forward movement of the screw and after the charging is completed, dwelling is performed.
The injecting process, as shown in FIG. 7, comprises a charging process for charging the resin into the cavity by a primary pressure and a dwelling process performed continuously by a secondary pressure, and generally, the charging process is performed under speed control, while the dwelling process is performed under pressure control. This is because an immediate release of a charging pressure causes the resin in a melted state to flow back from a gate, so that the back flow is to be prevented, and when the resin solidifies, it contracts, so that the contraction is to be compensated for.
Accordingly, since the dwelling is not required after the resin solidifies, the dwelling time is set to a time to reach the gate solidification, so that the dwelling time varies with the design dimensions of a molded article, and for a thick-wall molded article whose runner or gate is designed with thick-wall, inevitably the dwelling time is set to a longer value. After the dwelling time has elapsed, the process is shifted to the resin metering.
The back pressure at the metering is a pressure applied by a hydraulic pressure to the screw as a resistance to the backward movement of the screw associated with the forward movement of the resin for metering in a melted state, and a higher back pressure causes the screw backward moving speed to become slow and thus a plastication ability to be reduced, so that the back pressure is usually set to a value lower than the dwelling pressure.
The cavity cooling process is time counted from the dwelling completion, considered to be completed when the set time is reached, and then shifted to the next process such as a cavity opening and molded article taking out. The cooling time, which varies with the wall-thickness and shape of a molded article, is often generally longer than the metering time. However, although some cooling requires a shorter time, an excessive cooling has not particular effect on a molded article, so that in some case, after the cooling is performed to the metering completion, the metering process is shifted to the next process.
The improvement of the molding efficiency in such injection molding depends on how to shorten the molding cycle time, so that if the time required for any of sequential injection molding processes including clamping process and taking out process other than the process relating to injection can be shortened, the molding time would be shortened by the shortened time, thereby increasing the number of moldings. Although an effort such as the achieving of the high speed of each equipment in its own way has been effective to shorten the process time, the shortening of the molding time by improving equipment has a limit, so that current situation is such that the molding cycle shorter than current one cannot be expected.
If immediately after injection charging, the process is shifted to the dwelling so as to shorten time, a warpage, a sink mark and the like may partially occur in a molded article. Although the phenomenon may be prevented to some extent by the shape of a molded article or the design of a cavity gate, for a molded article subject to a limit in shape or gate design, there has been no preventive means.
Thus, in order to eliminate the partial occurrence of a warpage, a sink mark and the like, the present inventors have previously found a molding method comprising the steps of setting an injection pressure when the cavity is filled with a resin to zero value to perform the pressure relief of the resin in the cavity, and thereafter performing dwelling.
However, although the molding method has an advantage in that it can prevent the partial occurrence of a warpage and the like on a molded article without significantly changing molding process, it has a new problem in that a pressure relief time longer than conventional time is required, so that the molding time longer by that time difference is required.
The present invention has been made in view of the above-mentioned circumstances and it is an object of the invention to provide an injection molding method capable of shortening the time required from injection start to metering completion and making earlier the time count start point of cooling time, by integrating the dwelling process with the metering process, and capable of shortening the molding time even for a molding method requiring a pressure relief time in the injection process.